This story is part of a partnership between MedPage Today and VICE News.
Pope Francis arrives in Mexico on Friday for a highly anticipated tour of the country, but the timing is complicated by local reports of Zika virus infections, raising questions about whether the large crowds drawn to the pontiff will fuel an outbreak.
The pope's trip includes stops in Mexico City, Tuxtla Gutierrez, San Cristobal de Las Casas, Morelia, and Ciudad Juarez. The Centers for Disease Control and Prevention has already issued a level 2 travel advisory, warning visitors to the region to take precautions while urging women who are pregnant or who may become pregnant to stay away.
"Density of population is one factor that facilitates spread of infection," said Dr. Jeffrey Duchin, a professor of infectious diseases at the University of Washington and a fellow of the Infectious Diseases Society of America. "The more people are susceptible and crowded together, the more opportunity a disease would have theoretically to move from person to person."
Discovered in 1947, the Zika virus is spread by infected mosquitos. It was thought to only cause flu-like symptoms and a rash up until a few months ago. Soon after it was detected for the first time in Brazil in May 2015, it coincided with a rise in cases of a birth defect called microcephaly, which causes babies to be born with abnormally small heads and underdeveloped brains. Zika has also been linked to a rare paralysis condition called Guillain-Barre.
Although scientists are still investigating the relationship between the virus and the conditions, the World Health Organization declared the spread of the virues a public health emergency of international concern on February 1.
Zika has been reported in 25 countries and territories in the Americas, and it was first reported in Mexico in November, according to WHO. As of Feb. 3, the Mexican Ministry of Health had reported 36 confirmed Zika cases up from 18 the week before, according to Reuters.
This week, the CDC reported that Brazilian tissue samples from two babies with microcephaly who died shortly after their births and two miscarried fetuses had tested positive for the virus. They were all born to mothers who said that they had experienced Zika symptoms during pregnancy.
The CDC advisory regarding the papal visit is not an overreaction, said Dr. Robert Glatter, an emergency physician at Lenox Hill Hospital in New York. He noted that the mosquito species that carries Zika, called Aedes aegypti, is found in southern parts of the United States. Americans who visit Mexico to see Francis could be bitten and infected in Mexico and spread it to the local mosquito population upon return to the US, leading to wider infection.
"Anytime there is a mass gathering of international people going to one place, there's always a mixing of infectious agents. It puts people at risk," he said. Some medical analysts have suggested that the virus might have arrived in Brazil when it hosted the FIFA World Cup in 2014.
The virus is known to stay in the blood for between five and seven days. Only one in five people who are infected have symptoms, so it's possible for people to spread the virus without feeling sick.
Some experts say the papal visit won't necessarily have much of an additional impact, since Zika is expected to spread in Mexico over the coming months anyway.
"The virus will continue to make its way around the country on its own merit," remarked Dr. Frank Esper, a pediatric infectious disease specialist of UH Case Medical Center in Cleveland. "It doesn't need the pope's help."
Dr. William Schaffner, chair of preventive medicine at the Vanderbilt University Medical Center in Nashville, agreed, but added that it would be wise for health officials in Mexico to do as much as they can to limit the mosquito population as much as possible ahead of the pope's arrival. He said he's much more worried about the Olympics in Brazil this summer, which will draw even more people from all over the world.
Esper said it's also a comfort to remember that dengue fever and chikungunya, which are similar mosquito-borne diseases that are found in Central and South America, have not really gained a foothold in the United States, despite occasional outbreaks in places like Florida.
"It would be pretty hard for this Zika virus to truly become established in the United States," he said. "There's plenty of mosquito-borne diseases out there, but they haven't really been able to take root in the US because we have much better mosquito control."